This invention relates to a method for manufacturing a multilayer wiring board allowing interlayer connection with a bump, and more particularly to a technology for preventing sticking at the time of molding.
To manufacture a so-called build-up multilayer wiring board, it is necessary to laminate insulating layers and conductive layers one by one to pattern each of the conductive layers to a predetermined wiring pattern and to facilitate interlayer connection between the conductive layers. Consequently, technologies for forming a fine pattern on the conductive layers and for achieving effective interlayer connection become more important.
As a method for manufacturing the build-up multilayer wiring board, such a method has been known that bumps, which are provided to a copper foil, are buried in the insulating layer and thereafter the other copper foil is cemented onto the insulating layer to connect to the bumps (see, e.g., Patent Document 1).
The invention described in Patent Document 1 relates to a selective etching method and a selective etching apparatus both for forming the bumps. Patent Document 1 discloses technologies for manufacturing a multilayer wiring circuit board in which an etching barrier layer is provided to a main face of the copper foil to be provided with the bump; a member used for forming a wiring circuit board, which is provided with a copper foil used for forming a conductive circuit, is used as a base on a main surface of the etching barrier; and this member is processed as appropriate to obtain the multilayer wiring circuit board.
Through the technologies for manufacturing the multilayer wiring circuit board, first, the copper foil of the member used for forming the wiring circuit board is selectively etched to form the bumps for interlayer connection, and the intervals between the bumps are filled by the insulating layer to insulate each of the intervals. Next, the copper foil used for forming the conductive circuit is formed on upper surfaces of the insulating layer and the bumps. Subsequently, wiring films are formed by selectively etching the copper foils at both upper and lower surfaces. Consequently, the multilayer wiring board is formed, in which the wiring films are provided at both the upper and lower surfaces and the bumps connect the wiring films to each other.    Patent Document 1 is Japanese Patent Laid-Open Publication 2003-129259.
To manufacture the multilayer wiring board, it is necessary to cement the copper foil to the upper surfaces of the bumps through a molding process in which the resultant mass is sandwiched between stainless steel plates and pressurized therein to perform a thermocompression bonding to the copper foil.
Unlike a normal molding, this molding process requires application of high pressure to the extent that the bumps are squashed to some extent. This is because the copper foil only contacts the top surfaces of the bumps, but they need to be unified upon pressurization to the extent that the bumps are squashed so as to ensure a binding force. Consequently, a pressure applied in this molding is 90 to 150 kg/cm2, which is about three times as high as a pressure of about 35 to 40 kg/cm2 set in the normal molding. In the case of using a polyimide resin for the insulating layer, for example, not only a pressure but also a high temperature of about 335 degrees Celsius is applied.
Molding at a high temperature and a high pressure has a problem such as poor workability since the sticking occurs between the molded multilayer wiring board and each of the stainless steel plates especially at top portions of the bumps to make it difficult to take the product (the multilayer wiring board) off. It is difficult for an operator, for example, to disassemble the product by hand, thereby requiring some sort of tool, for example, to drive a wedge in gaps.
Where the sticking occurs, the product receives application of the force at the time of peeling, causing a fold, a wrinkle, ruggedness, distortion, a curl, and the like. The sticking occurs especially at the bump portions, leading to a big problem at an area that is densely packed with the bumps. Occurrence of wrinkling, ruggedness, or the like in the product enhances dimensional variation, which gets in the way of ensuring accuracy in patterning of wiring.
Furthermore, there is a possibility of causing a problem in intimate contact between the polyimide resin and the copper foil. For example, in molding by sandwiching the resultant mass between the stainless steel plates with a solid surface, the cemented copper foil is rigidly supported in a straight manner, thereby not being deformed along the lines of the bumps. As a result, the intimate contact between the cemented copper foil and the polyimide resin cannot be ensured, causing a so-called bleaching phenomenon.